Microelectromechanical devices, such as microstructures with movable elements (e.g. micromirror devices), have found many applications in basic signal transduction. For example, a spatial light modulator based on micromirror devices steers light in response to electrical or optical signals by deflecting the deflectable reflective mirror plates into different spatial directions. Such a spatial light modulator can be a part of a communication device or an information display.
A major factor that limits the reliability and widespread use of micromirror devices is adhesion. Adhesion is a result of the dominance of surface and interfacial forces, such as capillary, chemical bonding, electrostatic, and van der Waals forces, over mechanical forces which tend to separate micromirror device components. When mechanical restoring forces cannot overcome adhesive forces, the micromirror devices are said to suffer from stiction. Stiction failures in contacting micromirror devices, can occur after the first contacting event (often referred to as initial stiction), or as a result of repeated contacting events (often referred to as in-use stiction). Initial stiction is often associated with surface contamination (e.g., residues of bonding materials or photoresist), or with high energy of contacting surfaces (e.g., clean oxidized silicon or metallic surfaces). For the case of in-use stiction, each time one part of the micromirror (e.g. mirror plate of a micromirror device) touches the other (e.g. stopping mechanism) or the substrate, the contact force grows and ultimately becomes too large for the restoring force to overcome. In this case, the device remains in one state indefinitely. This phenomenon can arise from a variety of underlying mechanisms, such as contact area growth, creation of high-energy surface by micro-wear, surface charge separation etc.
The stiction of the micromirrors often exhibits dynamic characters. For example, the stiction in a micromirror can vary over time, and the restoration force necessary to overcome the stiction in the same micromirror may also vary over time. In a micromirror array device, such as a micromirror-based spatial light modulator, the stiction may occur in different micromirrors at different times in operation. Such stiction in individual micromirrors may also vary over time.
Another factor that may limit the usage of microelectromechanical devices with movable elements is the response delay or slow response of the external driving signals. For example, a micromirror device operates by deflecting the deflectable reflective mirror plate in response to the external electrostatic force. If the response time of the mirror plate to the external electrostatic force is too long, or the movements of the mirror plate is not instantaneous with changes of the external electrostatic force, the operation of the micromirror fails.
Therefore, what is needed is a micromechanical device with reliable and robust movable element.